Air circulation control systems for lumber dry kilns, wherein the air fans are all mounted on a common shaft, have generally required that the drive means (motor, steam turbine, etc.) which drives the fan shaft, be reversible so as to enable the flow of drying air to be reversed, thus ensuring a uniform drying process. This also has required that the fan blades be symmetrical, enabling equal air flow in either of the two directions. Reversible fan blades of this type inherently yield a lower efficiency (volume of air moved/power to drive fan) than the standard non-reversible type. Because of stored energy in this system ("flywheel effect"), the reversing process can take a matter of minutes to transpire, allowing the air flow to cease and causing the drying process to be prolonged. This results in a costly, inefficient dry kiln operation.